Although conventional glass vial-type thermometers are inexpensive and sufficiently accurate for most purposes, such thermometers present risks in the form of cross-contamination and ingestion of the thermal expansion fluid in the event of glass breakage. As this fluid is typically mercury, the risk is particularly dangerous. The advantages of tympanic temperature measurement are well known, and thermometers based upon this approach are becoming increasingly popular. With a tympanic thermometer, a thermal detector is used to sense the heat generated by the tympanic membrane (the eardrum) using a device having a speculum which fits into the ear canal to perform the measurement. The advantages of this technique are many, including comfort without the need for patient cooperation coupled with fast and accurate readings, assuming the instrument is properly designed.
The accuracy of a tympanic instrument is largely dependent upon the temperature sensing element exposed to the tympanic membrane, and, to a lesser extent, on the algorithm used to compute temperature given the level of thermal energy sensed by the detector element. In terms of non-contact tympanic temperature measurement, the most popular sensing elements include pyroelectric and thermopile detectors. Thermoscan, Inc. of San Diego, Calif., for example, now offers a tympanic thermometer based upon the pyroelectric effect.
More recently, thermopile-based detectors have gained in popularity as techniques have emerged to thermally stabilize such detectors; that is, to compensate for unwanted variations associated with the ambient environment. Yelderman et al (U.S. Pat. No. 5,159,936) "Non-Contact Infrared Tympanic Thermometer," provides a useful background concerning techniques used to thermally stabilize thermopile detectors in general, including approaches used in tympanic temperature measurement in particular.
As discussed in the Yelderman reference above, a thermopile compensation technique which has been known for some time involves the use of multiple thermocouple junctions, certain of which are exposed to the ambient environment in the immediate vicinity of the junctions, while other junctions are shielded from the radiating body to be measured, thus producing an output signal which is less susceptible to undesirable ambient variation. The specific implementations of Yelderman et al regarding this general approach leave certain issues unresolved, however. For one, it is unclear from this reference whether the multiple thermocouple junctions form part of a two-channel detector or a two-element detector. With a two-channel detector, a single thermopile is used and only one set of its junctions (hot or cold) are shielded from the incoming radiation so as to produce an output signal. With a two-element detector, on the other hand, two complete thermocouples are used, with the cold or reference junctions being thermally bonded to a common substrate, with only one set of the hot junctions being shielded with respect to the incoming radiation, resulting in a configuration which may be less susceptible to ambient variation.
Although the Yelderman patent makes reference to dual-element detectors, certain of the descriptions and figures teach away from the use of such a detector in the specific embodiment respecting tympanic temperature measurement, instead implying the use of a dual-channel implementation. For example, the detector in Yelderman is intentionally placed at the distal end of the speculum, presumably to position it in closer proximity to the tympanic membrane. Additionally, the shielding of one half of the sensing element is placed on the window of the element housing and distanced away from the thermocouple junctions. This increases the extent to which all junctions are exposed to the ambient environment and implies the use of a dual-channel as opposed to a dual-element sensor. Such a configuration also discourages the use of an optical guide or "light-pipe" between the temperature sensor and the tympanic membrane, first because Yelderman leaves no room for such an element, but additionally, the use of a light pipe would cause the radiation to be considerably more multidirectional than if derived directly from the tympanic membrane, which would result in pronounced leakage around the blocking element, thus leading to an erroneous reading. In certain applications, however, such a light-pipe may be essential to an accurate reading given the physiology of different individuals, particularly children. Additionally, Yelderman et al prescribes the use of a bandpass filter to pass only wavelengths in a range corresponding to emissions representative of the internal temperature of a human being. However, such a bandpass filter may be undesirable when particular calculations are used to derive a final temperature value. For example, if the integral of energy received is used as the basis for the calculation, band limiting causes the integral equation to remain dependent upon wavelength, which may result in inaccurate approximations. By receiving the total unimpeded radiation from the tympanic membrane, a more straightforward integral may be used which is independent of wavelength.